General Motors Corp. said it will lay off indefinitely 1,300 workers at its Buick City assembly plant in Flint, Mich., because of production cuts. GM spokesman John Mueller said the layoffs are effective Nov. 24. The No. 1 auto maker is cutting back production of its front-wheel-drive Buick LeSabre model. The work force at the plant is 6,700. GM said last week that it plans to close nine plants and curtail operations at two others by 1990, eliminating 29,000 jobs. Buick City layoffs were not part of that announcement.